1. Field of the Invention
The present invention relates to a display device (light emitting device) having a light emitting element, and more particularly such a display device having a display part of current driving method.
2. Description of the Related Art
Digital gradation method (digital driving method) and analog gradation method (analog driving method) can be given as driving method of multi-color expression on a display device having a light emitting element. In the aforementioned digital gradation method, a light emitting element is driven in binary of ON (brightness is almost 100%) and OFF (brightness is almost 0%) to obtain gradation by controlling the luminous region and the length of the period during which each pixel emits light. In the analog gradation method, analog input data is written into a light emitting element to modulate the gradation in an analog manner.
Furthermore, expression of gradation is made through two methods, which are a constant voltage drive which is dependent on the voltage applied to the light emitting element, and a constant current drive which is dependent on the current applied to the light emitting element. An electric current flowing through a light emitting element is controlled by a transistor (hereinafter referred to as driver transistor) in current drive.
Operation of a driver transistor is explained referring to the V-I feature shown in FIG. 8. There are two operating regions of the driver transistor, namely, a saturation region and a linear region.
Linear region is a region of which current value changes according to the voltage between the source and drain (Vds) and the voltage between the gate and source (Vgs). (|Vds|<|Vgs−VTh|) In the linear region, the following expression (1) is established. Note that Ids is the amount of current running through a channel forming region. Note also that β=μCo·W/L is established and μ thereof is a mobility of the driver transistor, Co is a gate capacity per unit volume, and W/L is a ratio of the channel width W to the length L in channel forming region.Ids=β{(Vgs−VTh)Vds−½·Vds2}  (1)
According to the expression (1) above, Vds and Vgs obtain the current value in the linear region. In the linear region, the lesser Vds becomes, the lesser current value becomes too, while the current value hardly increases even if Vgs gets larger.
When the driver transistor is operated in mainly the linear region, the amount of current flowing between both electrodes of the light emitting element is changed according to both values of Vgs and Vds. The driver transistor is used as a switch, and a power source line and the light emitting element are shorted if necessary, thereby flowing a current into the light emitting element. The current value flowing through the light emitting element is directly influenced by the characteristics (variation and deterioration in the manufacturing process) of the light emitting element that is connected to the driver transistor.
In the saturation region, the current value is not changed by the voltage between the source and drain (Vds), in other words, it is only changed by the voltage between the gate and source (Vgs). (|Vds|>|Vgs−VTh|)
In the saturation region, the following expression (2) is established.Ids=β(Vgs−VTh)2  (2)
As set forth in the expression (2), the current value in the saturation region is greatly dependent on a change in Vgs but not dependent on a change in Vds. Therefore, the current value in the saturation region is not influenced by the characteristics of the light emitting element connected to the driver transistor.
On the other hand, when the driver transistor is operated in mainly the saturation region, the amount of current flowing between both electrodes of the light emitting element is greatly dependent on a change in Vgs of the driver transistor but not dependent on a change in Vds. A gate voltage of the driver transistor is controlled to flow the necessary amount of current into the light emitting element. In other words, the driver transistor is used as a voltage control current source and the driver transistor is set such that a constant current flows between a power source line and the light emitting element.
In the constant current drive utilizing the abovementioned feature, the current value is not dependent on a change in Vds when the driver transistor is operated in the saturation region. Therefore, the amount of current flowing into the light emitting element can be constant regardless of the characteristics (variation in the manufacturing process, deterioration, and temperature variation) of the light emitting element.
When Vgs of a driver transistor is changed appropriately, the driver transistor can be operated in mainly a linear region or in mainly a saturation region.
Operating a driver transistor in the saturation region as shown above is disclosed in patent document 1.
[Patent Document]
Japanese Patent Laid-Open No. Hei 14-108285
In the abovementioned constant current drive, an operating region of a transistor steps into the linear region once Vds thereof is decreased to a certain point by the deterioration of an light emitting element. To avoid this, a setting voltage of Vds (Vds of a driver transistor in operation) is set with an estimated deterioration (voltage for deterioration, voltage α) of the light emitting element. The voltage a is dependent on the deterioration of the light emitting element.
In a conventional setting voltage, in short, Vds needed to be set high because of the estimated value (812) for the margin of the change in characteristics of a light emitting element between before (810) and after (811) deterioration. (|Vds|≧|Vgs−VTh+α|)
The voltage applied to the cathode and anode of a light emitting element thus became inevitably high, causing heat generation and high power consumption.
It is an object of the invention to provide a pixel structure which can be operated without adding the voltage a to the setting voltage for the deterioration of the light emitting element. Namely, a pixel structure with the setting voltage in the vicinity of the boundary between the saturation region and the linear region (813 in FIG. 8) is to be provided. A further object of the invention is to provide a display device provided with an aforementioned pixel and a control method thereof.